The present invention relates to methods and apparatus for recovering solvent vapor from an oven or the like and more particularly to such methods and apparatus wherein a material balance is maintained with respect to the oven atmosphere.
Prior art techniques for recovering solvent vapor from curing ovens during the curing of solvent borne resin coatings typically include the withdrawal of oven atmosphere and the condensation of solvent vapor therefrom. In certain processes, the oven atmosphere is comprised essentially of inert gas and solvent vapor and inert, noncondensed gas may be returned from the condensation unit to the oven for the purpose of inerting the oven. During such processes, internal oven conditions must be maintained such that solvent vapor remains in the oven atmosphere as the escape of solvent vapor presents environmental and safety hazards while the influx of excessive amounts of air may enable potentially explosive conditions to occur in the oven.
In addition to the foregoing requirement to maintain a material balance, it has been found that in order to provide for the efficient operation of a condensation unit designed to recover solvent vapor from an oven atmosphere comprised of such vapor and inert gas, it is preferred to supply the withdrawn atmosphere to a condensation unit at a substantially constant flow rate. An example of a preferred type of condensation or recovery unit is illustrated in U.S. Pat. No. 4,444,016 and which is assigned to the assignee of the present invention. A constant volume, rotary blower is preferably utilized as a means for supplying withdrawn atmosphere to the aforedescribed condensation unit and such blowers require that a substantially constant flow of gas be supplied thereto.
In prior art solvent recovery systems wherein inert gas is supplied to a curing oven, as for example is described in U.S. Pat. Nos. 4,150,494 and 4,337,582 both of which are assigned to the assignee of the present invention, conditions in the curing oven have been controlled in different modes. For example, in connection with the apparatus described in U.S. Pat. No. 4,150,494, the pressures existing in the oven and in the exit vestibule are detected and the flow of inert gas to the oven is controlled such that the pressure in the oven is maintained below that of the exit vestibule to assure that solvent vapor is retained in the oven and does not pass outwardly through the exit vestibule. However, in the event the oven pressure is less than a pre-set differential below the pressure in the entrance vestibule, the rate at which the oven atmosphere is withdrawn and is supplied to a condensation unit is increased to thereby effect a reduction of oven pressure. It has been found that the pressure differential existing between a vestibule and the oven interior is relatively small and that expensive, sophisticated instrumentation is required to reliably detect such pressures which is rendered even more difficult when large oven openings and relatively large gas flows exist. Consequently, it has been found in connection with such prior art solvent recovery systems that it is difficult if not virtually impossible to maintain a material balance with respect to the oven atmosphere. With regard to the system illustrated in U.S. Pat. No. 4,337,582, solvent vapor is precluded from condensing within an oven by assuring that the concentration of solvent vapor therein is maintained below a predetermined value. The actual solvent vapor concentration is monitored by sensing the dewpoint of such vapor in the oven and by assuring that such dewpoint remains below a predetermined dew point at which condensation would occur. This result is achieved by varying the rate at which solvent vapor is withdrawn from the oven such that upon detecting an increase in solvent vapor concentration, that is by sensing a dew point which approaches a predetermined dew point, the rate at which solvent vapor is withdrawn from the oven is increased thereby reducing the concentration of such vapor. Consequently, condensation of solvent vapor within the oven is precluded although a material balance with regard to the oven atmosphere is again difficult if not impossible to maintain with such prior art, solvent recovery systems. In addition to the requirement for controlling conditions in a particular curing oven, it is frequently desirable to connect a plurality of ovens to a single condensation unit as it may be economical to size such a single condensation unit to recover solvent vapor from a plurality of curing ovens. When a plurality of curing ovens are connected to a single condensation unit, it is important that variation of conditions in one oven does not affect other ovens. In the event one oven becomes "contaminated", that is, the oxygen content thereof exceeds a predetermined, safe level, it is important to be able to isolate such oven from the other ovens and the condensation unit so that the contaminated oven may be purged with a flow of inert gas and yet not impair the ability to recover solvent from other ovens which may be operating properly.
It will be appreciated by those skilled in the art that prior art techniques for controlling conditions within curing ovens to enable safe and effective operation thereof and the preferred operating conditions of condensation units are to some extent conflicting with one another. That is, oven control techniques have resorted to maximizing the rate at which oven atmosphere is withdrawn from the oven to enable lower temperatures to exist in the oven without causing vapor condensation in cold portions of the oven or on the material being cured, while solvent vapor condensation units require that minimum flow rates of oven atmospheres be supplied as feed streams to enable efficient condensation of solvent vapor. It will be understood that the efficiency of condensation units is essentially defined by the degree of refrigeration (i.e. liquid N.sub.2 and electrical power) required to condense a given unit of solvent vapor and thus, the extent to which such refrigeration consumed in recovering solvent vapor is minimized, the more efficient is the unit. In addition, the lower the gas flow is to the condensation unit, smaller and less expensive equipment may be employed. The prior art techniques mentioned above tend to emphasize the dichotomy between the requirements of oven control on the one hand and efficient condensation of solvent recovery on the other hand and such prior art does not suggest how these conflicting requirements can be simultaneously met by an oven control system. Consequently, the prior art reflects a clear need for methods and apparatus for controlling curing ovens during the recovery of solvent vapor by maintaining a material balance with respect to the oven atmosphere to assure safe and reliable oven operating conditions and yet compensate for variations in such conditions during oven operation.